A conventional open wrench is disclosed in U.S. Pat. No. 2,687,056 and comprises a head connected to an end of the handle and the head has a mounting hole which is defined through the head and an opening communicates with the front end of the head and the mounting hole. The top and bottom flat surfaces each have an inclined surface which is located around the mounting hole and the lowest area of the inclined surface is the at the opening so that the front end of the head is thin enough for convenience of inserting into a narrow space. As shown in FIG. 13, the head 40 of the conventional open wrench has an inclined surface which extends to the flat front end of the head 40. When the height of the opening of the front end has a standard height H1 which provides sufficient strength, however, the thickness of the head 40 at a small distance along the inclined surface becomes H4 which is so thick that requires much material and the head 40 will be too big to be inserted into the narrow space.
The present invention intends to provide an open wrench that can be inserted into a narrow space while the strength is maintained.